The type of surge arrestor that we are concerned with comprises a tubular porcelain housing, electrical terminals at opposite ends of the housing, and one or more stacks of metal-oxide varistors within the housing electrically connected between the terminals. When a voltage surge appears across the arrester, the varistors normally act in a conventional manner to pass surge currents through the arrester, thereby protecting any equipment shunted by the arrester from damage by the surge. Normally, the arrester can pass the surge current and any follow current without any electric arc being developed within the arrester housing. In the unusual event that a varistor should fail in service, an electric arc could rapidly develop within the housing alongside a stack of varistors, thereby abruptly generating very hot gases and resulting elevated pressures and temperatures within the housing.
For protecting against such internal pressures and temperatures, a typical arrester comprises means defining pressure-relief passageways leading to the terminals at opposite ends of the housing and venting means associated with the terminals for venting the arc-produced gases through the terminals. Such venting means is normally sealed by pressure-sensitive diaphragms or the like which rupture or otherwise operate in response to the elevated pressure to permit a rapid escape of the arc-generated gases. It is also conventional to utilize the escaping hot gases to transfer the arc from within the arrester housing to a pair of spaced electrodes located outside the housing and respectively connected to the terminals of the arrester. A rapid transfer of the arc from within the housing to an external location is highly desirable in limiting the pressure and temperature build-ups within the housing.
In certain circuit applications, if an internal arc such as above described should develop, the overvoltage surge arrester is subjected to an extremely high rate of energy input that can cause the pressure and temperature build-ups within the housing to rupture the porcelain housing, even despite the presence of conventional venting means for the arc-generated gases. One such circuit application is the use of the above type of surge arrester in series-capacitor compensation schemes. In such schemes, the surge arrester is connected across a series capacitor bank, and this parallel combination is connected in series with a power line. Should a varistor fail in such service, there is a likelihood that an arc will be established within the arrester housing, and the series capacitor bank will discharge through this internally-located arc, developing a very high current (typically 300-400 kA peak) with a high frequency (typically 2,000-3,000 Hz); and this will usually be coincident with a high 60 Hz current (typically 10-40 kA rms) from the line source. This combination of currents imposes an extremely high rate of energy input on the arrester early in the failure event that, unless effectively protected against, can result in rupture of the porcelain housing of the arrester in a violent manner.
In seeking to solve this problem, we have conducted tests, using as test samples arresters in which the housing was constructed of a higher strength porcelain than standard strength porcelain. In some of these test samples, we have left the bore of the housing unlined, and in others we have lined the bore with a layer of silicon rubber intended to thermally shield the porcelain housing from the high-temperature arc, as well as to mechanically shield it from any shrapnel (such as pieces of the failed varistor disks) that might strike it. These test samples failed when subjected to the above-described high currents. With regard to the latter test samples, it appears that the silicon rubber liner ablates rapidly in the presence of the high-current arc and actually adds to the internal gas pressure.
We also tested a representative arrester in which the housing bore was provided with a Teflon liner. Teflon was selected because it is a material often used in the presence of arcs, as it generates when exposed to an arc a gas that lowers the arc temperature and cools the housing walls. This test sample also failed when subjected to the above-described high currents.